Cytokinin-facilitated proteolysis of ARABIDOPSIS RESPONSE REGULATOR 2 attenuates signaling output in two-component circuitry

Cytokinins propagate signals via multiple phosphorelays in a mechanism similar to bacterial two-component systems. In Arabidopsis, signal outputs are determined by the activation state of transcription factors termed type-B Arabidopsis response regulators (ARRs); however, their regulatory mechanisms are largely unknown. In this study, we demonstrate that the proteolysis of ARR2, a type-B ARR, modulates cytokinin signaling outputs. ARR2-hemagglutinin (HA) is rapidly degraded by cytokinin treatment, but other type-B ARRs, such as ARR1-HA, ARR10-HA, ARR12-HA and ARR18-HA, are not. ARR2 degradation is mediated by the 26S proteasome pathway, and requires cytokinin-induced phosphorylation of Asp80 residue in the receiver domain. Through mutational analysis of amino acid residues in the receiver domain, we found that substitution of Lys90 with Gly inhibits ARR2 degradation. ARR2K90G-HA in transgenic Arabidopsis conferred enhanced cytokinin sensitivity in various developmental processes, including primary root elongation, callus induction, leaf senescence and hypocotyl growth. ARR2K90G-HA increased the expression of type-A ARRs, primary cytokinin-responsive genes and indicators of signaling output in two-component circuits. Expression of ARR2K90G-HA from the native ARR2 promoter in the arr2-4 knock-out mutant also increased cytokinin sensitivity. In conclusion, ARR2 proteolysis is involved in the maintenance of the primary signaling output for normal developmental processes mediated by cytokinin in Arabidopsis.